1. Field of the Invention
The present invention relates to a planar circuit for use in a resonator, an oscillator, an antenna, and the like in high frequency bands such as a microwave band, and more particularly, to a planar circuit which is suitable for enhancing the functionality and facilitates the manufacturing to achieve a reduction in cost.
2. Description of the Related Art
A variety of planar circuits based on the microwave integrated circuit technology are used in, for example, wireless communication system apparatuses and measuring apparatuses for implementing resonators, oscillators, antennas, and the like, and the demand for the planar circuits tends to increase year by year. In recent years, these planar circuits are also required to provide higher functionality and facilitate the manufacturing from a viewpoint of achieving higher performance of a variety of devices and improving the productivity.
FIGS. 1A and 1B are a plan view and a cross-sectional view, respectively, of a resonator which comprises a typical conventional planar circuit. Here, the figures illustrate an oscillator which utilizes a resonator.
The resonator comprises circuit conductor 2 having a circular plane shape shaped on one main surface of substrate 1 made of a dielectric material, and ground conductor 3 formed on the entirety of the other main surface of substrate 1. This type of resonator is called the xe2x80x9cplanar resonatorxe2x80x9d. In this configuration, a TM mode resonator is formed by an electric field generated between circuit conductor 2 and ground conductor 3 and a magnetic field associated therewith. As appreciated, the oscillation frequency at which the oscillator operates depends on dielectric coefficient E and thickness d of substrate 1, and area S of circuit conductor 2.
The circuit illustrated in FIGS. 1A and 1B further comprises Gunn diode 6 embedded in substrate 1, which forms the planar resonator, to make up an oscillator. Generally, Gunn diode 6 is placed within a through hole formed through substrate 1, and both ends of Gunn diode 6 are connected to circuit conductor 2 and ground conductor 3, respectively, for example by soldering. Then, Gunn diode 6 functions as an amplifier for oscillation to implement an oscillator circuit for amplifying and feeding back a resonant frequency component of the resonator. The resonator serves for phase synchronization and power combination as well as functions as a resonant element of the oscillator circuit. Lead line 4 extending from circuit conductor 2, and capacitive line 5 branched lead line 4 for matching are also disposed on the one main surface of substrate 1.
FIGS. 2A and 2B are a plan view and a cross-sectional view, respectively, of a planar antenna, i.e., an antenna which comprises a planar circuit.
This planar antenna comprises circuit conductor 2 having, for example, a square plane shape on one main surface of substrate 1; ground conductor 3 having an opening 7 as an intermediate layer of substrate 1; and a conductive line serving as signal line 8 on the other main surface of substrate 1. In this configuration, a planar resonator is formed by circuit conductor 2 and ground conductor 3, while a microstrip line is formed by signal line 8 and ground conductor 3. The microstrip line forms a high frequency transmission line by the action of an electric field generated between signal line 8 and ground conductor 3, and a magnetic field associated therewith. Thus, an antenna is implemented for transmitting or receiving electromagnetic waves at a resonant frequency of the planar resonator and fed through the microstrip line. The planar resonator is electromagnetically coupled to the microstrip line through opening 7 extending through ground conductor 3 which is provided as an intermediate layer.
However, both planar circuits based on a planar resonator configured as described above experience difficulties in changing the electric characteristics, so that the resonant frequency is obliged to remain fixed. It is therefore difficult to design the planar circuit to generate a variable resonant frequency, by way of example, thereby providing a highly functional planar circuit. Specifically, the electric characteristics of a planar resonator depend on a fixed electromagnetic wave field based on a boundary condition defined by a geometric shape, a connection condition of input and output lines, component materials, and the like. Thus, difficulties exist in designing a variable planar circuit. In essence, it can be said that current planar circuits, i.e., planar resonators still remain unchanged from simple utilization of the electromagnetic wave field fixedly set by given conditions.
Also, as illustrated by the exemplary configuration of the planar oscillator using Gunn diode 6, conventionally, a circuit element comprising a semiconductor device including an integrated circuit and an active circuit, a positive element, and the like is typically embedded in substrate 1 between circuit conductor 2 and ground conductor 3 and connected to conductors 2, 3. Such a configuration requires a step of piercing substrate 1 in the manufacturing, thereby giving rise to problems of making highly accurate manufacturing difficult to cause a lower productivity, and making the circuit element incompatible with surface mounting.
For disposing a circuit element on a surface of a substrate, for example, for disposing the aforementioned Gunn diode 6 on the one main surface of substrate 1, circuit conductor 2 must be connected to ground conductor 3 through a so-called via-hole, i.e., electrode through hole. In this event, however, the via hole causes an increase in the conductor length or line length to increase an inductance component, resulting in deteriorated high frequency characteristics. Therefore, conventionally, the circuit element is embedded in subrtrate 1 to reduce the line length for preventing the high frequency characteristics from being deteriorated.
It is an object of the present invention to provide a planar circuit which is capable of variably controlling an electromagnetic wave field to significantly improve the functionality and performance, and achieving a higher productivity resulting from the ease of manufacturing.
The object of the present invention is achieved by a planar circuit which includes a substrate, a planar circuit conductor disposed on a first main surface of the substrate, a ground conductor disposed on a second main surface of the substrate for creating a high frequency resonator based on an electromagnetic wave field in cooperation with the planar circuit conductor, and a circuit element disposed on an opening formed in at least one of the planar circuit conductor and the ground conductor, and connected to the at least one conductor along the periphery of the opening to electronically control the electromagnetic wave field.
Used as the circuit element may be a variable reactance element such as a varactor diode, and a switching element such as a PIN diode, Schottky barrier diode and the like, and furthermore an MMIC (monolithic microwave integrated circuit) and the like. Such a circuit element is connected to the planar circuit conductor or the ground conductor at least two points along the periphery of the opening.
In the present invention, an opening is formed in at least one of the planar circuit conductor and ground conductor which makes up a planar resonator, and the circuit element is disposed in the opening for electronically controlling the electromagnetic wave field. As a result, the boundary condition for the planar circuit can be equivalently changed to change the electromagnetic wave field, thereby realizing a variable planar circuit. Since the circuit element is disposed in the opening formed on one main surface of the substrate, surface mounting can be employed for mounting the circuit element to facilitate the manufacturing. When a variable reactance element is used as the circuit element, the characteristic of the electromagnetic wave field can be changed to vary the resonant frequency. Alternatively, when a switching element is used as the circuit element, the switching element can be controlled ON/OFF to select the resonant condition of the planar resonator to be operative or inoperative. In addition, when the planar resonator is designed to have degenerated resonance modes and a switching element is used as the circuit element, one of the degenerated resonance modes can be selected.